Method of connecting circuit boards

ABSTRACT

The present invention provides a circuit board connecting method that includes an alkane application step and a heat-press-bonding step. The alkane application step is a preliminary step which applies an alkane group to a printed circuit board. The heat-press-bonding step heat-press-bonds a flexible circuit board to the printed circuit board by positioning their printed wire terminals and conductive thick-film terminals to face one another. In the heat-press-bonding step, the alkane group boils and cleans the surface of the printed wire terminals and conductive thick-film terminals, thereby removing the oxide film to expose the metallic portion of the terminals. The alkane group soaks into a thermoplastic film or base plate, causing it to swell and thereby bond the flexible circuit board to the printed circuit board tightly. Accordingly, both electrical and mechanical firm bonding is accomplished at a very low cost and short time.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present invention is related to Japanese patent applicationNo. Hei. 2000-94206, filed Mar. 30, 2000; 20002-35493, filed Aug. 3,2000; 11-368006, filed Dec. 24, 1999, the contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a method of bonding metal, andmore particularly, a method of connecting the wiring on two circuitboards.

BACKGROUND OF THE INVENTION

[0003] In Japanese unexamined patent application publication Heisei8-330726, a method of connecting the metal wiring on two circuit boardsis disclosed, in which board electrodes are bonded together by solderdue to the fact that the oxide film formed on the surface of the solderis broken down by the dilation energy of a boiling hydrocarbon compound.

[0004] However, when using the dilation energy of a hydrocarboncompound, even if the oxide film on the metal surfaces of the solderfused can be broken down at the time of bonding, the oxide film on thesurfaces of the base metal composing the board electrodes cannot beremoved because this base metal will not be fused. Thus, when solder isapplied to only one side of a board electrode, sufficient bondingstrength cannot be obtained. Presently, methods exist for using apress-bond connector which press-bonds and fixes two circuit boards anda method of using ACF (anisotropic conductive film). However, the methodof using a press-bond connector does not prevent increased cost of theconnector and an increased space for connection. The method of using theACF, which is based on the point contact of conductive particles, doesnot prevent increased connection resistance and is also uncertain of theconductive reliability of the connecting section.

[0005] For dealing with this matter, there is disclosed a method ofusing insulating adhesive in JP-A No. S60-140896 referred to here asprior art 1, and a method of using conductive adhesive in JP-A No.H9-320662 referred to here as prior art 2.

[0006] However, both of the above-mentioned prior art references useadhesive and therefore take time for fixing regardless whether it is hotmelt type or thermosetting adhesive. Therefore, these methods are notcapable of bonding boards in a short time and necessitate much work forconnection. Specifically, prior art 1 takes 20 seconds forheat-press-bonding of the connecting section.

[0007] Moreover, none of the prior art performs extremely low-costbonding. Particularly, prior art 1 is designed to bond metallic lugshaving a special shape which protrude from the printed circuit board,and therefore it is difficult to connect for low cost.

[0008] Moreover, insulating adhesive or thermoplastic resin used forbonding may flow into the terminal connecting section, resultingpossibly in faulty conduction. The use of conductive adhesive can causeshort-circuiting between adjacent terminals. Accordingly, none of theprior art references is sufficiently reliable for connection betweenterminals.

SUMMARY OF THE INVENTION

[0009] Accordingly, the object of the present invention is to provide amethod of bonding metal in which sufficient bonding strength can beobtained.

[0010] Another object of the present invention is to provide a method ofconnecting a printed circuit board and a flexible circuit board, themethod being capable of connecting these members at low cost and in ashort time, while having the sufficient reliability.

[0011] In a first aspect of the invention, a hydrocarbon compound inwhich the energy of disassociation of the C—H bond is less than 950kJ/mol is interposed between the connecting portion of the wiring on afirst and a second circuit board. By heating the hydrocarbon compound,the hydrocarbon compound is decomposed and a radical is formed in whichhydrogen has been separated from the hydrocarbon compound. Bondingoccurs as the oxide film formed on the surface of the metal is reducedby this radical. Here, the C—H bond disassociation energy Delta H, asshown in FIG. 6, is the energy necessary for the alkyl group and thehydrogen to disassociate while the hydrocarbon compound retains each oftheir electrons, and is calculated after the electron orbit of eachcompound is determined. In other words, the C—H bond disassociationenergy Delta H of each compound is the ease in which alkyl groups andhydrogen on the hydrocarbon compound can be disassociated. The smallerthis compound's energy is, the easier it is for alkyl groups andhydrogen to disassociate.

[0012] Then, as shown in FIG. 6, when the alkyl group and hydrogendisassociate while retaining each of their electrons, that alkyl groupbecomes a radical, takes away oxygen from copper oxide or the like, orin other words, reduces the copper oxide, and turns into a stable alkaneoxide compound. In this way, sufficient bonding strength can by obtainedby using a hydrocarbon compound which demonstrates a reducing action bymeans of its heat decomposition.

[0013] In another aspect of the invention, a circuit board connectingmethod comprises using an alkane application step of applying an alkanegroup to at least the surface of a printed circuit board where printedwire terminals exist or a portion of the surface of a flexible circuitboard where conductive thick-film terminals exist, and aheat-press-bonding step of bonding the flexible circuit board to theprinted circuit board by heat-pressing, while positioning the printedwire terminals and the conductive thick-film terminals to face oneanother.

[0014] The base plate of the printed circuit board has an epoxy glassboard not confined. For example, resin boards based on other resinexcluding multiple boards and multiple materials can be used, or otherceramic circuit boards, etc. can also be used. The printed wireterminals are typified by a printed pattern of copper foil, etc. that isnot confined. For example, a gold or silver foil, a gold-platedconductor, or a conductive paste, etc. called a conductive thick filmcan be used.

[0015] The flexible circuit board is also called flexible printedcircuit board, and is a flexible and plastic printed circuit board. Thethermoplastic resin for forming the film which is the base of theflexible circuit board is typified by PEN (polyethylene naphtalatehaving a fusing point of around 270-280° C.), but it is not confined.For example, PET (polyethylene terephthalate having a fusing point ofaround 340° C.), PEEK (polyether ketone having a fusing point of around340° C.), or PPS (polyphenylensulfide having a fusing point of around250° C.) can be used.

[0016] The conductive thick-film terminals of the flexible circuit boardare typified by conductive paste such as silver paste, but it is notconfined, and they may be formed of a metallic foil, etc. The conductivepaste can be gold paste, aluminum paste, copper paste, etc. besides thesilver paste.

[0017] The alkane group applied can be any proper saturated hydrocarbon,even though it is not a straight-chain type, and the alkane group mayeven include an impurity which is harmless for bonding. Alkane which isthe main component of the alkane group preferably has a boiling pointwithin a proper range lower than the fusing point of the thermoplasticresin which forms the film of the flexible circuit board. The ones whichare somewhat outside the range are still usable. Even materials whichslightly differ from alkane groups and have constituents other than themethyl group, such as alcohol groups or ether groups, can be used.However, materials other than alkane group develop polarity, whichresults in the generation of ions, and therefore the use of alkanegroups is still desirable to prevent short-circuits.

[0018] This means initially carries out an alkane application step forthe preliminary step, and thereafter proceeds to a heat-press-bondingstep for the main bonding step so that a flexible circuit board isconnected to the printed circuit board.

[0019] In the initial alkane application step, alkane group is appliedto at least either the portion of the surface of the printed circuitboard where printed wire terminals exist or the portion of the surfaceof the flexible circuit board where conductive thick-film terminalsexist.

[0020] In the ordinary bonding process, in which a printed circuit boardis placed and a flexible circuit board is bonded upside down to it, itis preferable to have the application step for the printed circuit boardwhich faces upward. There is no restriction on the manner ofapplication, and it can be the use of a brush or roller means or it canbe spray application. Accordingly, the alkane application step can befinished in a short time.

[0021] The alkane application step is the preliminary step for theheat-press-bonding step explained next, and it is solely intended to puta small amount of alkane group on the bonding surface of the printedcircuit board or flexible circuit board prior to the heat-press-bondingstep.

[0022] At the subsequent heat-press-bonding step, the flexible circuitboard is heat-press-bonded to the printed circuit board, with theprinted wire terminals and the conductive thick-film terminals beingpositioned to face one another-. For heat-press-bonding the flexiblecircuit board to the printed circuit board, a heating tool such as aheated metallic block is brought in press-contact with the flexiblecircuit board, or the flexible circuit board and printed circuit boardin a state of press-contact are subjected to ultrasonic heating.

[0023] When the flexible circuit board is heat-press-bonded to theprinted circuit board in the heat-press-bonding step, the alkane groupwhich has been applied in advance acts in two ways as follows.

[0024] Firstly, the alkane group is heated to a temperature above theboiling point to boil instantaneously, cleaning the surface of theprinted wire terminals of the printed circuit board and the surface ofthe conductive thick-film terminals of the flexible circuit board sothat both members can be easily bonded. Specifically, boiling removesthe oxide film formed on the surface of the printed wire terminals ofthe printed circuit board, causing the metallic portion which is notoxidized inside the printed wire terminals to be exposed. Similarly,boiling removes the contaminant which covers the surface of theconductive thick-film terminals of the flexible circuit board, causingthe metallic portion of the conductive thick-film terminals to beexposed.

[0025] The alkane group, which is saturated hydrocarbon, is low in itsC—H bond-dissociation energy and has some reducing action. Therefore, inthe formation of metal oxide on the surface of the printed wireterminals and conductive thick-film terminals, the alkane group reducesthe oxide back to metal. As a result, the oxide is thoroughly removedfrom the surface of the printed wire terminals and conductive thick-filmterminals, causing their metallic surface to be exposed.

[0026] Consequently, the metallic portion of the printed wire terminalsand the metallic portion of the conductive thick-film terminals areexposed and press-bonded to one another by being in direct contact at ahigh temperature. As a result, both members are bonded firmly to oneanother, which not only achieves a strong mechanical bond, but alsoachieves the satisfactory conduction based on the firm electricalconnection.

[0027] secondly, the heated alkane group soaks into the material (epoxyglass, etc.) which forms the film of the flexible circuit board or thebase plate of the printed circuit board, causing the material to swell.Consequently, the thermoplastic resin which forms the film is heated tofuse and swell, sealing the space between adjacent terminals andsticking firmly to the surface of the base plate between the printedwire terminals on the surface of the printed circuit board and the sideface of the printed wire terminals. As a result, the printed circuitboard and the flexible circuit board are bonded firmly to have anenhanced strength against peeling, and both members are firmly bondedmechanically. Moreover, the film seals the portion between terminals,preventing the short-circuiting and erosion caused by emerging dew.

[0028] Namely, the heat-press-bonding step not only firmly connects theprinted wire terminals and the conductive thick-film terminalselectrically and mechanically, but it also bonds the printed circuitboard and the flexible circuit board mechanically. The sealed bondingsection prevents short-circuiting caused by ions between adjacentterminals and also prevents the short-circuiting and erosion caused byemerging dew. As a result, the reliability of connection between theprinted circuit board and the flexible circuit board is improved.

[0029] Also, thermoplastic resin which forms the film of the flexiblecircuit board swells to fill the space of the bonding section and sealthe bonding section, making the bonding section to hardly develop theshort-circuiting or defective connection due to emerging dew, whereby ahigh reliability of connection is achieved. Also, the circuit boardconnecting scheme based on this means achieves the sufficient connectionreliability.

[0030] Further areas of applicability of the present invention willbecome apparent from the detailed description provided hereinafter. Itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, areintended for purposes of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

[0032]FIG. 1 is a cross-sectional view explaining the mounting method inthe first embodiment of the present invention;

[0033]FIG. 2 is a cross-sectional view for explaining a mounting methodaccording to the present invention;

[0034]FIG. 3 is a cross-sectional view for explaining a mounting methodaccording to the present invention;

[0035]FIG. 4 is a cross-sectional view for explaining a mounting methodaccording to the present invention;

[0036]FIG. 5 is a cross-sectional view for explaining a mounting methodaccording to the present invention;

[0037]FIG. 6 is a figure for explaining the copper oxide reductionreaction by an alkane;

[0038]FIG. 7 is a figure showing the relation between the C—H bonddisassociation energy and the reduction rate constant according to thepresent invention;

[0039]FIG. 8 is a figure for explaining the method of measuring thecopper oxide reduction rate according to the present invention;

[0040]FIG. 9 is a figure showing the relationship between C—H bonddisassociation energy and the connection surface moment according to thepresent invention;

[0041]FIG. 10 Cross-sectional view for explaining the mounting method ofthe second embodiment according to the present invention;

[0042]FIG. 11 is a cross-sectional view showing the mounting method ofthe second embodiment according to the present invention;

[0043]FIG. 12 is a cross-sectional view showing the mounting method ofthe second embodiment according to the present invention;

[0044]FIG. 13 is a cross-sectional view showing the mounting method ofthe second embodiment according to the present invention;

[0045]FIG. 14 is a cross-sectional view showing the mounting method ofthe second embodiment according to the present invention;

[0046]FIG. 15 is a cross-sectional view showing the mounting method ofthe second embodiment according to the present invention;

[0047]FIG. 16 is a cross-sectional view showing the mounting method ofthe second embodiment according to the present invention;

[0048]FIG. 17 is a cross-sectional view showing the mounting method ofthe second embodiment according to the present invention;

[0049]FIG. 18 is a cross-sectional view showing the mounting method ofthe second embodiment according to the present invention;

[0050]FIG. 19 is a cross-sectional view showing the mounting method ofthe second embodiment according to the present invention;

[0051]FIG. 20 is a perspective view showing the initial state of theheat-press-bonding step based on the present invention;

[0052]FIG. 21 is a cross-sectional diagram showing the principalarrangement of the heat-press-bonding step based on the presentinvention;

[0053]FIG. 22 is a cross-sectional diagram showing the intermediatestate of the heat-press-bonding step based on the present invention;

[0054]FIG. 23 is a cross-sectional diagram showing the bonded stateafter the heat-press-bonding step based on the present invention;

[0055]FIG. 24 is a perspective view showing the principal arrangement ofa multiple circuit board made based on the present invention;

[0056]FIG. 25 is a graph showing an effect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0057] Below, the first embodiment of this invention will be explainedin accordance with the figures.

[0058] In FIG. 5, a second circuit board 20 is mounted on top of a firstcircuit board 10, and shows the connection between the wiring of each.Specifically, it shows the state after a connection has been completed.In the present embodiment, the first circuit board employs a printedwiring board (PWB) thereon. This circuit board 10 includes an insulationsubstrate 11, on the surface of which is formed a metal wiring connectorC1. The metal wiring connector C1 is composed of a metal electrode 12composed of copper. The second circuit board 20 employs a flexiblewiring board (FWB) thereon. This circuit board 20 includes an insulationsubstrate 21, on the surface of which is formed a metal wiring connectorC2. The metal wiring connector C2 is composed of a metal electrode 22composed of copper, and solder 23, which is attached to and covers metalelectrode 22.

[0059] In this type of embodiment, the metal wiring connectors C1 and C2of the first and second circuit boards are composed of metal electrodes12 and 22 respectively, and are configured such that at least one sideof metal electrode 22 is placed and attached on top of at least one sideof metal electrode 12 by means of solder 23. Then, metal wiringconnector C1 on the first circuit board 10 is bonded to metal wiringconnector C2 on the second circuit board (the copper terminal and thesolder coated copper terminal are bonded together). In this way, metalwiring connector C1 of the first circuit board and metal wiringconnector C2 of the second circuit board are connected together.

[0060] Next, FIGS. 1-5 will be used to explain the method of production.First, as shown in FIG. 1, first circuit board (PWB) 10 and secondcircuit board (FPC) 20 are prepared. At this time, the surface of metalwiring connector (copper wiring) C1 on the first circuit board 10 has anoxide film formed thereon due to air oxidation. Further, the surface ofsolder 23 on metal wiring connector C2 on the second circuit board 20has an oxide film formed thereon due to air oxidation.

[0061] Then, as shown in FIG. 2, a hydrocarbon compound 30 with a C—Hbond disassociation energy lower than 950 kJ/mol is applied to metalwiring connector (copper wiring) C1 on the first circuit board 10. Thishydrocarbon compound can be at least one selected from the groupconsisting of cyclooctane, tetramethyl pentadecane, triphenyl methane,dicyclopentadiene, or dihydroanthracene.

[0062] Continuing, as shown in FIG. 3, the second circuit board 20 isplaced on top of the first circuit board 10 such that metal wiringconnectors C1 and C2 are opposite one another. By this means, thehydrocarbon compound 30 with a C—H bond disassociation energy lower than950 kJ/mol can be interposed between the metal composing metal wiringconnector C1 on the first circuit board and the metal composing themetal wiring connector C2 on the second circuit board, and the metalwiring connectors C1 and C2 on both boards can be disposed opposite eachother.

[0063] Then, when pressure is applied between metal wiring connectors C1and C2 on both boards 10 and 20, the solder 23 is heated above itsmelting point. The amount of pressure applied is, for example, 0.3 to2.0 MPa. Further, pressure and heating is applied from 1 to 10 seconds.

[0064] At this time, as shown in FIGS. 4 and 6, by heating hydrocarboncompound 30, hydrocarbon compound 30 is decomposed, and hydrocarboncompound 30 has hydrogen detached therefrom and turns into a radical.While reducing the oxide film 12 a and 23 a formed on the metal surfacesby means of the radicalized hydrocarbon compound 30, the metal composingthe metal wiring connectors C1 and C2 on both boards is bonded togetherby means of the fusing of metal (solder). In other words, the oxide film12 a and 23 a is removed by the reduction of oxide film 12 a and 23 a,and a clean metal surface is exposed. In the state in which wetness isgood, the surface of copper film 12 on board 10 contacts with thesurface of solder 23 on board 20. Moreover, as shown in FIG. 5,accompanied by the melting of solder 23, solder 23 on board 20 is bondedtogether with copper film 12 on board 10. In this type of embodiment, byheating hydrocarbon compound 30 to a temperature above the melting pointof solder 23, the oxide film 12 a and 23 a on the surfaces of metalelectrode 12 or solder 23 is reduced by hydrocarbon compound 30 as themetal electrodes 12 and 22 on both boards bond together by fusing withsolder 23.

[0065] In this way, the oxide film 12 a and 23 a on the surfaces of thebase metal (in the present example, copper film 12) and solder 23 areeliminated, the base metal and solder 23 are connected, and a highlyreliable connection between component packages (the connection betweenthe wiring) becomes possible. In other words, by using hydrocarboncompound 30 (a hydrocarbon compound in which the disassociation energyof the C—H bond is less than 950 kJ/mol) to reduce the oxide films 12 aand 23 a on the surfaces of the metal, clean metal surfaces can becontacted and fixed together with solder, and a good connection withhigh reliability can be obtained.

[0066] In the present embodiment of this type, by using the reducingaction exhibited by hydrocarbon compound 30, suitable bond strength canbe obtained. In other words, by using a specific hydrocarbon compoundwith a C—H bond disassociation energy less than 950 kJ/mol ashydrocarbon compound 30, the hydrocarbon compound can be made to exhibita reducing action.

[0067] The present inventors have conducted various experiments inregard to bonding theory, and explain this as follows.

[0068] (i) The levels of hydrogen and water generated when copper oxidewas soaked and heated in every species of liquid hydrocarbon compoundwere determined. The results were that the occurrence of hydrogen wasconfirmed, but water was not detected. Because of this, it was confirmedthat the reduction of copper oxide cannot be accomplished by means ofhydrogen.

[0069] (ii) The reaction product generated when copper oxide was soakedand heated in every species of liquid hydrocarbon compound was analyzed.The results were that the presence of oxidized hydrocarbon compounds wasconfirmed (for example, in the case of cyclooctane, the presence ofcyclooctanone and cyclooctanol were confirmed). By this means, it wasdetermined that there is a possibility that hydrocarbon compounds reducecopper oxide.

[0070] (iii) To confirm whether the conclusion drawn in (ii) wascorrect, the relationship between the C—H bond disassociation energy andthe reduction rate constant was sought. The results of this are shown inFIG. 7. The horizontal axis of FIG. 7 is the C—H bond disassociationenergy Delta H, and the vertical axis is the reduction rate constant.The samples used were dicyclopentadiene, triphenylmethane, cyclooctane,tetramethylpentadecane, and eicosane. Here, as shown in FIG. 8, asubstrate (an oxidized copper electrode) is placed into a trial subjectand heated for a fixed period at 300 degrees Centigrade, the oxygen onthe surface of the copper electrode is analyzed with dispersedwavelength X-ray spectroscopy, and the reduction rate constant isdetermined by means of the following formula.

Reduction rate constant=(1−X/X1)/(ttimesX)

[0071] Where:

[0072] X1 is the X ray count in the first oxidation state

[0073] X is the X ray count at each interval of elapsed time

[0074] T is time of heating (seconds)

[0075] These results, as shown in FIG. 7, confirm the relation that thesmaller the C—H bond disassociation energy becomes, the larger thereduction rate. Because of this, it was confirmed that copper oxide andthe like is reduced by hydrocarbon compounds that have become radicals.

[0076] Using a species substance in which the C—H bond disassociationenergy is relatively low at the point where bonding between the copperterminal and the solder covered terminal occurs, as shown in FIG. 9,with respect to a hydrocarbon compound with a C—H bond disassociationenergy of less than 950 kJ/mol, the prior art flux and the equivalentconnection surface moment can be obtained, and sufficient bondingstrength can be ensured (the smaller the C—H bond disassociation energy,the more favorable the bonding characteristics). The details are shownin FIG. 9, in which the horizontal axis is the C—H bond disassociationenergy Delta H, the vertical axis is the connection surface moment, andthe samples used were dihydroanthracene, dicyclopentadiene, cyclooctane,tetramethylpentadecane, and eicosane. Here, with respect to theconnection surface moment, if one were to make (hypothetically) a squareobservation window using the short end of the rectangle forming thebonding area, and bring said observation window to the area in therectangular bonding area in which the bonding is worst, the window'scontents will face the entire area and the actual bonding area ratiodemanded. The results are that in order to have a connector surfacemoment above 0.7 when using a flux, it is understood that it isdesirable to use a substance having a C—H bond disassociation energy ofless than 950 kJ/mol.

[0077] In addition, with respect to bonding without using flux, insoldering, previously flux was used and after bonding was cleaned up.However, because cleaning became difficult because of environmentalproblems, and because a problem was created in which flux residue causesa decline in insularity, when soldering and not using flux, the effectof oxide on the connection area does not ensure a sufficient connection,and connection reliability is poor. Thus, it is useful to not use fluxand to break down the oxides in order to ensure good connectivity.Further, in the present method, a flux-like metal is not melted, andthere is no decrease in insularity so as to not create metal ionactivity. In other words, the reduction reaction of the present methoddoes not create metal ions so that oxygen can be pulled away from theoxide.

[0078] Next, a second embodiment will be explained with emphasis on thepoints of difference with the first embodiment.

[0079] In the first embodiment, solder is used to bond the copperterminal and the solder coated copper terminal together. However, in thepresent embodiment, the metals composing both terminals are bondedtogether by mutual dispersion.

[0080] In FIG. 14, a second circuit board 20 is mounted on top of afirst circuit board 10, and the wiring on each is in the connectedstate. In other words, FIG. 14 shows the state after connection.

[0081] A metal wiring connector C1 is formed on the surface of aninsulating substrate 11 on the first circuit board (PWB) 10. A metalwiring connector C1 is composed of a metal electrode 12 formed fromcopper, a nickel film 13 attached to and covering the surface of thecopper electrode 12, and a gold film 14 formed on top of the nickel film13.

[0082] A metal wiring connector C2 is formed on the surface of aninsulating substrate 21 on the second circuit board (FPC) 20. A metalwiring connector C2 is composed of a metal electrode 22 formed fromcopper, and a tin film 23 attached to and covering the surface thereof.

[0083] Then, the gold film 14 on the first circuit board 10 and the tinfilm 23 on the second circuit board 20 are bonded together by the mutualdiffusion of both metals. In this way, the metal wiring connector C1 onthe first circuit board 10 is connected with the metal wiring connectorC2 on the second circuit board.

[0084] Next, the method of production will be explained with the use ofFIGS. 10-14.

[0085] First, as shown in FIG. 10, first circuit board (PWB) 10 andsecond circuit board (FPC) 20 are prepared. At this time, an oxide film14 a is formed by air oxidation on the surface of gold film 14 on themetal wiring connector C1 of first circuit board 10. Further, an oxidefilm 24 a is formed by air oxidation on the surface of tin film 24 onthe metal wiring connector C2 of second circuit board 20.

[0086] Then, as shown in FIG. 11, a hydrocarbon compound with a C—H bonddisassociation energy less than 950 kJ/mol is applied on top of goldfilm 14 of the first circuit board 10.

[0087] Continuing, as shown in FIG. 12, the second circuit board 20 isplaced on top of the first circuit board 10 such that metal wiringconnectors C1 and C2 oppose each other. By this means, metal wiringconnectors C1 and C2 of both boards 10 and 20 are disposed opposite oneanother in a state in which hydrocarbon compound 30 is interposedbetween the metal (14) of the first circuit board 10 and the metal (23)of the second circuit board 20. In other words, metal wiring connectorsC1 and C2 of both boards 10 and 20 are disposed opposite one another ina state in which hydrocarbon compound 30 having a disassociation energyless than 950 kJ/mol is interposed between the metal composing metalwiring connector C1 on the first circuit board 10 and the metalcomposing metal wiring connector C2 on the second circuit board 20.

[0088] Then, while applying pressure between metal wiring connectors C1and C2 of both boards 10 and 20, the metal (gold) 14 on the firstcircuit board 10 and the metal (tin) 23 on the second circuit board 20is heated to below the melting point of tin. The amount of pressureapplied is, for example, 0.3 to 2.0 MPa. Further, the heatingtemperature is between 180 and 200 degrees Centigrade, lower than themelting point of tin (232 degrees Centigrade). Moreover, the time inwhich pressure is applied and heating occurs is between 1 and 10seconds.

[0089] At this time, as shown in FIG. 13, by heating hydrocarboncompound 30, hydrocarbon compound 30 decomposes, and hydrocarboncompound 30 forms a radical by disassociating hydrogen therefrom. Whilereducing the oxide film 14 a and 23 a formed on the surface of metal bymeans of the hydrocarbon compound radical, metal wiring connectors C1and C2 on both boards 10 and 20 bond by means of the diffusion of metal.

[0090] That is, the oxide film 14 a on the surface of gold film 14 andthe oxide layer 23 a on the surface of tin film 23, in other words, theoxide layer 14 a and 23 a on the surface of metal 14 and 23 on the firstand second circuit boards 10 and 20, is reduced by means of hydrocarboncompound 30. The oxide film 14 a and 23 a is removed by the reduction ofoxide film 14 a and 23 a, and a clean metal surface is exposed. In thestate in which wetness is good, the surface of gold film 14 on board 10contacts with the surface of tin film 23 on board 20. Then, the mutualdiffusion of gold and tin takes place, and as shown in FIG. 14, tin 23on board 20 is bonded together with gold film 23 on board 10.

[0091] In the present embodiment of this type, when metal wiringconnector C1 on the first circuit board 10 contains gold, metal wiringconnector C2 on the second circuit board 20 contains tin, andhydrocarbon compound 30 is interposed between this gold and tin, byheating to below the melting point of tin, both metals can bond by meansof diffusion.

[0092] In this way, the oxide films 14 a and 23 a on the surface of thebase metals (in this embodiment, gold film 14 and tin film 23) iseliminated and the base metals bond, and a highly reliable connectionbetween parts packages (the connection between the wiring) becomespossible. In other words, by using hydrocarbon compound 30 to reduce theoxide films 14 a and 23 a on the surface of the metals, clean metalsurfaces can be contacted and mutual diffusion can occur, and a goodconnection with high reliability can be obtained.

[0093] Further, because there is no soldering process, the means ofbonding is inexpensive. Moreover, in the case of soldering, there is anelectrode pitch limit of 0.3 mm. However, if the present method is used,bonding can occur at an electrode pitch lower than 0.3 mm. In otherwords, the solder mounting method is unsuitable for minute connections,and in the alloy method, because alloy spills out beyond its intendedarea, it is also unsuitable for minute connections. However, by usingthe present method, it can be applied to products that have a minutepitch. More particularly, in recent years, the demand on the electrodepitch of component packages and connector technology has risen. However,the solder mounting method is unsuitable for minute connections, and inthe soldering method, there is an electrode pitch limit of 0.3 mm.Further, in the same way, in the alloy method, because the alloy spillsout beyond its intended area, it is also unsuitable for minuteconnections. However, the present embodiment can be applied to productshaving a minute pitch.

[0094] As above, even in the case where the electrode space is small, ahigh connection reliability in component packages (the connectionbetween wiring) can be carried out.

[0095] Third Embodiment

[0096] Next, a third embodiment will be explained with emphasis on thepoints of difference with the second embodiment.

[0097] In FIG. 19, a second circuit board 60 is mounted on top of afirst circuit board 50, and the wiring on each is in the connectedstate. In other words, FIG. 14 shows the state after connection.

[0098] In the first circuit board 50, metal wiring connector C1 isformed on the surface of insulation substrate 51. Metal wiring connectorC1 is composed of metal electrode 52 and is constructed of copper.Further, an alumina substrate is used for insulating substrate 51. Inthe second circuit board 60, metal wiring connector C2 is formed on thesurface of insulation substrate 61. Metal wiring connector C2 iscomposed of metal electrode 52 and is constructed of copper Further, analumina substrate is used for insulating substrate 61.

[0099] Then, metal wiring connector (copper wiring) C1 of first circuitboard 50 and metal wiring connector (copper wiring) C2 of the secondcircuit board 60 are bonded together by mutual diffusion. In this way,metal wiring connector C1 of first circuit board 50 and metal wiringconnector C2 of second circuit board 60 are connected together.

[0100] Next, the method of production will be explained by using FIGS.15 to 19.

[0101] First, as shown in FIG. 15, first circuit board 50 and secondcircuit board 60 are prepared. At this time, in first circuit board 50,oxide film 52 a is formed on the surface of metal wiring connector(copper wiring) C1 by air oxidation. Similarly, in second circuit board60, oxide film 62 a is formed on the surface of metal wiring connector(copper wiring) C2 by air oxidation.

[0102] Then, as shown in FIG. 16, a hydrocarbon compound having a C—Hbond disassociation energy less than 950 kJ/mol is applied on top ofmetal wiring connector (copper wiring) C1 of first circuit board 50. Thehydrocarbon compound is at least one selected from the group consistingof cyclooctane, tetramethylpentadecane, tryphenylmethane,dicyclopentadiene, and dihydroanthracene.

[0103] Continuing, as shown in FIG. 17, second circuit board 60 isplaced on top of first circuit board 50 such that metal wiringconnectors C1 and C2 oppose one another. In the state in whichhydrocarbon compound 70 is interposed between metal wiring connector C1of first circuit board 50 and metal wiring connector C2 of secondcircuit board 60, metal wiring connectors C1 and C2 of both boards 50and 60 are disposed opposite one another. In other words, in the statein which a hydrocarbon compound 70 having a C—H bond disassociationenergy less than 950 kJ/mol is interposed between the metal composingmetal wiring connector C1 of first circuit board 50 and the metalcomposing metal wiring connector C2 of second circuit board 60, metalwiring connectors C1 and C2 of both boards 50 and 60 are disposedopposite one another.

[0104] Then, in the state in which pressure is applied between metalwiring connectors C1 and C2 on both boards 50 and 60, the metal (copper)composing the metal wiring is heated to below its melting point. Thepressure applied at this time is, for example, 0.3 to 2.0 MPa. Further,the heat applied is between 700 and 1000 degrees Centigrade, lower thanthe melting point of copper (1083 degrees Centigrade). Moreover,pressure and heat is applied between 30 and 60 seconds.

[0105] At this time, by heating hydrocarbon compound 70, hydrocarboncompound 70 is decomposed and hydrocarbon compound 70 has hydrogenseparated therefrom to form a radical. Oxide film 52 a and 62 a formedon the metal surfaces is reduced at the same time the metal composingmetal wiring connectors C1 and C2 on both boards bond together bydispersion due to the radical formed from the hydrocarbon compound.

[0106] That is, the oxide film 52 a on the surface of copper film 52 andthe oxide film 62 a on the surface of copper film 62 a, in other words,the metal oxides 52 a and 62 a on the metal surfaces composing the metalwiring, are reduced by hydrocarbon 70. Clean metal surfaces are exposedby the reduction of oxides 52 a and 62 a. As a result, as shown in FIG.18, the state of wetness is good, and the surface of copper film 52 onboard 50 contacts with the surface of copper film 62 on board 60. Then,mutual dispersion of both coppers occurs (both coppers mutually dispersein the hard phase), as shown in FIG. 19, copper film 52 on board 50 isbonded together with copper film 62 on board 60.

[0107] In this manner, the oxide films 52 a and 62 a on the surfaces ofthe base metal (in the present embodiment, copper film 52 and 62) aredegraded and the surfaces connected, resulting in the high connectionreliability of component packages. In other words, by using hydrocarbon70 as an oxidizer on oxide films 52 a and 62 a on the metal surfaces,both clean metal surfaces can contact, mutual dispersion can occur, anda favorable connection with high reliability can be obtained.

[0108] As above, similar with the second embodiment, even when theelectrode space is small, a high connection reliability can occur incomponent packages (the connection between wiring).

[0109] In addition, in the aforementioned first through thirdembodiments, the hydrocarbon compound is only applied to metal wiringconnector C1 on the first circuit board. However, it can be applied onlyto metal wiring connector C2 of the second circuit board or to bothmetal wiring connectors C1 and C2 on both boards.

[0110] The circuit board connecting method as provided in a fourthembodiment of the present invention resides in a method of bondingtogether a printed circuit board 1 having a base plate 11 and aplurality of printed wire terminals 12 which are bonded to the surfaceof the base plate 11, and a flexible circuit board 2 having a film 21made of thermoplastic resin and a plurality of conductive thick-filmterminals 22 which are bonded to the surface of the film 21 as shown inFIG. 20. The circuit board connecting method of this embodiment is amanufacturing method of manufacturing a multiple circuit board 100having a printed circuit board 1 and a flexible circuit board 2connected together as shown in FIG. 24 by connecting the printed wireterminals 12 and the conductive thick-film terminals 22 correspondinglywhen the printed circuit board 1 and the flexible circuit board 2 arebonded.

[0111] The base plate 11 of the printed circuit board 1 is a multiplecircuit board made of base material of epoxy glass as shown in FIG. 21,and each printed wire terminal 12 is a connecting terminal of a printedwire made of copper foil formed on one surface of the base plate 11. Theprinted wire terminals 12 have their surfaces covered with a thin solderlayer 13 called solder leveler. Solder forming the solder layer 13 iseutectic solder, having a fusing point of about 183° C.

[0112] After the alkane application step which will be described later,i.e., before the heat-press-bonding step, the printed circuit board 1 tobe bonded to the flexible circuit board 2 has its surface covered with athin layer of liquid alkane group 3. The alkane group 3 employed in thisembodiment is tetradecane which is straight-chained saturatedhydrocarbon including 14 carbon atoms in a molecule and has a boilingpoint of 174° C. Namely, the boiling point of the alkane group 3 may belower than the fusing point of thermoplastic resin (PEN) which forms thefilm 21 of the flexible circuit board 2 as will be described next.

[0113] The film 21 of the flexible circuit board 2 including PEN(polyethylene naphthalate) and is a thermoplastic film which is thinnerincomparably than the base plate 11 of the printed circuit board 1 andhas a fusing point of about 270-280° C. The conductive thick-filmterminals 22 are wire terminals which are made from a thick film ofsilver paste and formed by printing of polyester resin including a largequantity of fine silver powder.

[0114] The circuit board connecting method of this embodiment includesan alkane application step and a heat-press-bonding step.

[0115] The alkane application step is a preliminary step which appliesthe above-mentioned alkane group 3 to the portion of the surface of theprinted circuit board 1 where the printed wire terminals 12 exist. Aprescribed amount of the alkane group 3 is applied with a brush to theend face of the printed circuit board 1 to cover the range of bondingshown in FIG. 20.

[0116] The heat-press-bonding step heat-press-bonds the flexible circuitboard 2 to the printed circuit board 1 with a heating tool H bypositioning the printed wire terminals 12 and the conductive thick-filmterminals 22 to face one another as shown in FIG. 20. The heating tool His made of titanium having a shape of a bar with a square cross section.It is heated to a temperature of 230-240° C. and driven at a pushingforce of around 2 MPa (about 20 kgf/cm²) to heat-press the flexiblecircuit board 2 to the printed circuit board 1 for 5 seconds. The film21 of the flexible circuit board 2 is relatively thin, and therefore itheats up nearly to the temperature of the heating tool H.

[0117] Accordingly, in the heat-press-bonding step, the film 21 reachesthe maximum temperature of around 230° C., lower than the fusing point(about 275° C.) of the thermoplastic resin (PEN) which forms the film 21and higher than the boiling point (174° C.) of tetradecane as the alkanegroup 3. Therefore, the film 21 softens but does not melt, whereas thealkane group 3 boils. The eutectic solder which forms the solder layer13 has a fusing point (183° C.) which is virtually equal to the boilingpoint (174° C.) of tetradecane as the alkane group 3. Therefore, itheats sufficiently in the heat-press-bonding step so that the solderlayer 13 undergoes fusion when the alkane group 3 boils.

[0118]FIG. 20 and FIG. 21 show the initial state of theheat-press-bonding step. FIG. 22 shows the intermediate state amid theheat-press-bonding step, and FIG. 23 and FIG. 24 show the bonded stateafter the heat-press-bonding step. As shown in FIG. 23 and FIG. 24, theflexible circuit board 2 is tightly bonded at its bonding section to theprinted circuit board 1 in the bonded state after the heat-press-bondingstep.

[0119] (Effect of Embodiment 4)

[0120] The circuit board connecting method of this embodiment arrangedas described above attains the following effectiveness. When theflexible circuit board 2 is heat-press-bonded to the printed circuitboard 1 in the heat-press-bonding step described above, the alkane group3 which has been applied in advance acts in two ways as follows.

[0121] Primarily, as shown in FIG. 22, the alkane group 3 boilsinstantaneously by being heated to a temperature above its boilingpoint, cleaning the surface of the printed wire terminals 12 of theprinted circuit board 1 and the surface of the conductive thick-filmterminals 22 of the flexible circuit board 2. As such, both members 12and 22 are bonded easily. At this time, eutectic solder which forms thesolder layer 13 has melted and is liquid due to heating to a temperatureabove its fusing point.

[0122] Consequently, by boiling, the alkane group 3 removes the oxidefilm formed on the surface of the solder layer 13 covering the printedwire terminals 12 of the printed circuit board 1. At the same time,oxides formed on the surface of the solder layer 13 are reduced tometals by the reducing action of the alkane group 3. Accordingly, thealkane group 3 exposes the metallic portion of the solder layer 13 notoxidized, and melts to cover the printed wire terminals 12. Similarly,because of boiling and the reducing action, the alkane group 3 removesthe oxide film and contaminant which covers the surface of theconductive thick-film terminals 22 of the flexible circuit board 2. Thisexposes the metallic portion of the conductive thick-film terminals 22.

[0123] As a result, the metallic portion of the solder layer 13 whichcovers the printed wire terminals 12 and the metallic portion of theconductive thick-film terminals 22 are exposed and press-bonded by beingin direct contact with one another at a temperature high enough to meltthe solder layer 13. As a result, the printed wire terminals 12 and theconductive thick-film terminals 22 are firmly solder-bonded, which notonly achieves strong bonding mechanically, but also achievessatisfactory conduction based on firm electrical connection.

[0124] Secondary, as shown in FIG. 22 again, the heated alkane group 3soaks into the film 21 of the flexible circuit board 2, causing the film21 to melt and swell slightly. Namely, thermoplastic resin which formsthe film 21 not only increases in fluidity by the slight melting as thetemperature rises, but it swells to seal the space between adjacentprinted wire terminals 12. Consequently, the film 21 of the flexiblecircuit board 2 firmly bonds the surface of the base plate 11 betweenthe printed wire terminals 12 on the surface of the printed circuitboard 1 and the side face of the printed wire terminals 12 as shownagain in FIG. 23. As a result, the printed circuit board 1 and theflexible circuit board 2 are bonded firmly to enhance the strengthagainst peeling, and both members 1 and 2 are firmly bondedmechanically. Moreover, swelling and filling film 21 blocks emergingdew, preventing the short-circuiting and erosion caused by dew.

[0125] Namely, the heat-press-bonding step not only firmly connects theprinted wire terminals 12 and the conductive thick-film terminals 22electrically and mechanically, but also it firmly bonds the printedcircuit board and the flexible circuit board mechanically. Moreover, thebonding section is sealed tightly by the film 21. Consequently, theshort-circuiting caused by ions between adjacent terminals 12 and 22 isprevented, and the short-circuiting and erosion caused by emerging dewis also prevented. As a result, the reliability of connection betweenthe printed circuit board 1 and the flexible circuit board 2 isimproved.

[0126] At the end of the heat-press-bonding step of this embodiment, theflexible circuit board 2 is bonded to the printed circuit board 1 asshown in FIG. 24, and a multiple circuit board 100, with the printedwire terminals 12 of the printed circuit board 1 and the conductivethick-film terminals 22 of the flexible circuit board 2 being connectedcorrespondingly, is manufactured.

[0127] As a result of the above, a printed circuit board 1 and aflexible circuit board 2 can be heat-press-bonded by merely applying anextremely inexpensive alkane group 3 to the bonding section of theprinted circuit board 1. Consequently, the printed circuit board 1 andthe flexible circuit board 2 can be connected at an extremely lowmaterial cost.

[0128] Also, the alkane application step can be finished instantaneouslyand the heat-press-bonding step can be finished in only 5 seconds,whereby the printed circuit board 1 and the flexible circuit board 2 canbe connected efficiently in extremely short time and with little work.

[0129] Moreover, the bonding of the printed wire terminals 12 andconductive thick-film terminals 22 is electrically and mechanicallyfirm. Also, the film 21 of the flexible circuit board 2 melts and swellsto stick firmly to the printed circuit board 1, whereby the reliablebonding can be achieved. In addition, thermoplastic resin which formsthe film 21 of the flexible circuit board 2 melts slightly into thealkane group 3. Also, the alkane group 3 soaks and swells to fill thespace of the bonding section. As a result, the film 21 protrudes to fillthe bonding section, and the bonding section does not develop theshort-circuiting or defective connection due to emerging dew, wherebythe reliable connection is achieved. Lastly, the circuit boardconnecting method based on this embodiment is sufficiently reliable interms of connection, while being capable of connecting a printed circuitboard 1 and a flexible circuit board 2 at an extremely low cost and in ashort time.

[0130] (Embodiment 5)

[0131] The circuit board connecting method as embodiment 5 of thisinvention differs from the foregoing embodiment 4 in that the printedcircuit board 1 does not have the solder layer 13 and that the alkanegroup 3 used is saturated hydrocarbon which mainly composed ofcyclooctane. The remainder of the circuit board connecting method ofthis embodiment is identical to embodiment 4 inclusive of the variousconditions of the heat-press-bonding step. Cyclooctane, which is themain component of the alkane group 3, is saturated hydrocarbon having aclass 3 carbon structure and a boiling point of about 148° C.

[0132] The printed wire terminals 12 of the printed circuit board 1 areexposed, and the heat-press-bonding step initially connecting theprinted wire terminals 12 of the printed circuit board 1 and theconductive thick-film terminals 22 of the flexible circuit board 2directly. As such, the alkane group 3 having a low boiling point boilsharshly to clean the surface of the printed wire terminals 12, andtherefore the printed wire terminals 12 and the conductive thick-filmterminals 22 are firmly connected even in the absence of the solderlayer 13. The alkane group 3 exerts the reducing action also in thisembodiment to remove the oxide film formed on the surface of the printedwire terminals 12 and conductive thick-film terminals 22 thereby toexpose their metallic portions. This aids both members 12 and 22 tobecome bonded. Consequently, the printed wire terminals 12 and theconductive thick-film terminals 22 are also reliably electricallyconnected.

[0133] Next, the heated alkane group 3 soaks into the film 21 of theflexible circuit board 2, causing the film 21 to have an increasedfluidity so that it seals the bonding section of the printed circuitboard 1 and flexible circuit board 2 tightly.

[0134] (Embodiment 6)

[0135] The circuit board connecting method as embodiment 6 of thisinvention is a method of connecting a printed circuit board 1, which hasprinted wire terminals 12 but does not have a solder layer nor platinglayer as in the case of the embodiment 5, and a flexible circuit board 2which has conductive thick-film terminals 22 formed of silver paste.

[0136] The arrangement of this embodiment is basically the same as thearrangement of the embodiment 5, but is different from the embodiment 5in the use of dicyclopentadiene for the alkane group. Dicyclopentadieneis an alkane group having class 3 carbon bond, with its C—Hbond-dissociation energy AH being estimated to be about 916 kJ/mol asshown in FIG. 7. The C—H bond-dissociation energy ΔH is defined to be anenergy level at which the alkane group undergoes thermal decompositionto release hydrogen thereby turning to the radical state as shown inFIG. 6.

[0137] Specifically, in this embodiment, the alkane group has a C—Hbond-dissociation energy of about 916 kJ/mol, which is below 950 kJ/mol.The film of thermoplastic resin which forms the flexible circuit board 2acts to seal the connecting section of the printed circuit board andflexible circuit board in the heat-press-bonding step.

[0138] Since, in this embodiment, the alkane group has a C—Hbond-dissociation energy of about 916 kJ/mol which is below 950 kJ/mol,the reduction speed constant k (refer to FIG. 7) is large, and it exertsa strong reducing action for metallic oxides at a high temperature. As aresult, metallic oxides which are liable to emerge on the surface of theprinted wire terminals 12 and conductive thick-film terminals 22 arereduced, causing both terminals 12 and 22 to have a metal-to-metaljunction. Consequently, even if a solder layer or plating layer isabsent on the surface of the printed wire terminals 12, it not onlyenhances the mechanical bonding strength between both terminals 12 and22, but also reduces the resistance of connection between both terminalsto achieve the satisfactory electrical connection.

[0139] Specifically, the peeling strength of this embodiment usingdicyclopentadiene (DCPD) doubles as compared with the case of theabsence of alkane group or the case of the alkane group of decane (C10)as shown by the mark of filled circle in FIG. 25. In addition, theconnection resistance between both terminals 12 and 22 decreasesdrastically as compared to the absence of alkane groups or the alkanegroup of decane (C10) as shown by the mark of blank circle in FIG. 25.

[0140] Accordingly, the circuit board connecting method of thisembodiment further increases the bonding strength between the printedcircuit board 1 and the flexible circuit board 2 in addition to theeffect of the preceding embodiment 5. Moreover, it has the effect offurther reducing the resistance of connection between the printed wireterminals 12 and the conductive thick-film terminals 22 and achievesmuch better electrical connection. Obviously, these effects are attainedeven without the surface processing for the printed wire terminals 12 aswhere in the preceding embodiment 5,

[0141] It is possible to alter the material of the printed wireterminals 12 or the material of the conductive thick-film terminals 22in carrying out the circuit board connecting method. For example, theconductive thick-film terminals 22 may be formed of a paste of copper,silver-copper alloy or tin instead of the silver paste. The printed wireterminals 12 may undergo nickel-gold plating, copper-silver plating,copper-lead plating, or the like. Dicyclopentadiene as the alkane group3 may be replaced with other alkane group having a C—H bond-dissociationenergy of 950 kJ/mol or less. Any of these variant embodiments achievesthe effect comparable to the above embodiment.

[0142] While the above-described embodiments refer to examples of usageof the present invention, it is understood that the present inventionmay be applied to other usage, modifications and variations of the same,and is not limited to the disclosure provided herein.

1. a method of connecting a first wiring connector on a first circuitboard and a second wiring connector on a second circuit board,comprising: disposing said first wiring connector on said first circuitboard and said second wiring connector on said second circuit board,said first wiring connector disposed opposite said second wiringconnector, said first wiring connector on said first circuit boardcomposed of metal and said second wiring connector on said secondcircuit board composed of metal and having a hydrocarbon compoundinterposed therebetween, said hydrocarbon having a C—H bonddisassociation energy less than 950 kJ/mol; and, decomposing saidhydrocarbon compound to form a radical, said hydrocarbon beingdecomposed by separating hydrogen from said hydrocarbon compound byheating said hydrocarbon compound, said radical formed from saidhydrocarbon compound and reducing oxide film on a surface of said metalof said first wiring connector and said second wiring connector whilebonding the first wiring connector and second wiring connector on saidfirst circuit board and said second circuit board respectively by fusionor diffusion.
 2. The method according to claim 1, wherein said firstwiring connector and second wiring connector on said first and secondcircuit boards respectively are metal electrodes, solder being placed ontop of and attached to at least said first wiring connector or saidsecond wiring connector.
 3. The method according to claim 2, whereinsaid first wiring connector and said second wiring connector bondtogether by fusing with said solder while said oxide film on the surfaceof said solder or said metal electrode is reduced by heating saidhydrocarbon compound above a melting point of said solder.
 4. The methodaccording to claim 1, wherein said first wiring connector and saidsecond wiring connector are made of copper.
 5. The method according toclaim 1, wherein said first wiring connector on said first circuit boardis comprised of gold, said second wiring connector on said secondcircuit board is comprised of tin, said hydrocarbon compound isinterposed between said gold and tin, and both metals are bondedtogether by diffusion by heating below a melting point of tin.
 6. Themethod according to claim 1, wherein said hydrocarbon compound is atleast a member of a set selected from the group consisting ofcyclooctane, tetramethylpentadecane, tryphenylmethane,dicyclopentadiene, and dihydroanthracene.
 7. A method of bonding metal,comprising: a method of connecting a metal first wiring connector on afirst circuit board and a metal second wiring connector on a secondcircuit board, comprising: disposing first wiring connector and secondwiring connector on said first circuit board and said second circuitboard respectively, said first wiring connector being disposed oppositesaid second wiring connector, metal on said first circuit board andmetal on said second circuit board have a hydrocarbon compoundinterposed therebetween, said hydrocarbon having a C—H bonddisassociation energy less than 950 kJ/mol; and applying a pressure tosaid metal first wiring connector and second wiring connector on saidfirst circuit board and said second circuit board; heating said metal onsaid first circuit board and said metal on said second circuit board toa point lower than a lowest melting point of said metals to decomposesaid hydrocarbon compound to form a radical by separating hydrogen fromsaid hydrocarbon compound due to said heating of said hydrocarboncompound, said radical formed from said hydrocarbon compound reducingoxide films on a surface of said metals while bonding said metals bymeans of mutual diffusion.
 8. A method for bonding metal, wherein ametal first wiring connector on a first circuit board and a metal secondwiring connector on a second circuit board are connected, the methodcomprising: disposing said metal first wiring connector and metal secondwiring connector on said first circuit board and said second circuitboard opposite one another, said first wiring connector on said firstcircuit board and said second wiring connector on said second circuitboard having a hydrocarbon compound interposed therebetween, saidhydrocarbon having a C—H bond disassociation energy less than 950kJ/mol; and applying pressure to said metal first wiring connector andsecond wiring connector on said first board and said second board,heating metal of the first wiring connector and second wiring connectorto a point lower than a melting point of said metal to decompose saidhydrocarbon compound and form a radical by separating hydrogen from saidhydrocarbon compound by means of heating said hydrocarbon compound, saidradical reducing oxide films on a surface of said metal while bondingboth of said metals by mutual diffusion.
 9. A method of manufacturing amultiple circuit board having a printed circuit board and a flexiblecircuit board which are connected together, the printed circuit boardhaving a base plate and a plurality of printed wire terminals which arebonded to a surface of the base plate and the flexible circuit boardhaving a film of thermoplastic resin and a plurality of conductivethick-film terminals which are bonded to the surface of the film, themethod comprising: applying an alkane group to at least a portion of asurface of the printed circuit board where the printed wire terminalsexist or a portion of a surface of the flexible circuit board where theconductive thick-film terminals exist in an alkane application step; andheat-press-bonding the flexible circuit board to the printed circuitboard in a heat-press-bonding step, said heat-press-bonding occurringwhile positioning the printed wire terminals and the conductivethick-film terminals to face one another.
 10. A circuit board connectingmethod according to claim 9, wherein the alkane group is composed mainlyof saturated hydrocarbon including between 8 and 20 carbon atoms in amolecule.
 11. A circuit board connecting method according to claim 9,wherein the alkane group has a boiling point lower than the fusing pointof the thermoplastic resin which forms the film.
 12. A circuit boardconnecting method according to claim 11, wherein, in theheat-press-bonding step, the maximum temperature of the film is lowerthan the fusing point of the thermoplastic resin which forms the filmand higher than the boiling point of the alkane group.
 13. A circuitboard connecting method according to claim 11, wherein the printedcircuit board has a solder layer that includes solder having a fusingpoint lower than the boiling point of the alkane group and covers theprinted wire terminals.
 14. A circuit board connecting method accordingto claim 9, wherein the alkane group is composed mainly of groups havingC—H bond-dissociation energy of 950 kJ/mol or less.
 15. A circuit boardconnecting method according to claim 9, wherein the film seals theconnecting section between the printed circuit board and the flexiblecircuit board at the heat-press-bonding step.